Various injection devices exist that employ an automated mechanism to actuate injection of a liquid medicament into a patient. Examples of such devices include jet injectors (both needle-free and needle-assisted) and traditional, low-pressure auto-injectors (that provide, for example, mechanized delivery of a traditional, finger-powered hypodermic syringe injection). Although the precise mechanisms used to complete an injection can vary, most include a feature that stores kinetic energy that can be used to drive an injection mechanism during use. Further, many injectors include a trigger mechanism configured to ensure that the kinetic energy remains stored until an injection is desired, whereby actuation of the trigger releases the injection mechanism, allowing the stored kinetic energy to drive the injection mechanism to cause injection.
Examples of needle-free jet injectors are described, for example, in U.S. Pat. Nos. 5,599,302 and 4,790,824. These high force injectors are button activated and administer medication as a fine, high velocity jet delivered under sufficient pressure to enable the jet to pass through the skin. The injection mechanism in such needle-free jet injectors can apply a force to a medicament storing chamber within the device such that the pressure required to inject the medicament is created within the chamber.
Traditional self-injectors or auto-injectors like the ones described, for example, in U.S. Pat. Nos. 4,553,962 and 4,378,015 and PCT Publications WO/9929720 and WO/9714455 inject medicament at a rate and in a manner similar to hand-operated hypodermic syringes. The described self-injectors or auto-injectors have needles that are extended at the time of activation to penetrate the user's skin to deliver medicament through movement of the drug container and related needle. Thus, the mechanism that provides the force to deliver the medicament in traditional, low-pressure self-injectors and auto-injectors can also be used to extend the needle and displace the drug container to cause the insertion of the needle through the user's skin and to apply a force to a plunger movably disposed within the drug container to cause the medicament to be expelled from the container through the needle. The auto-injectors manufactured, for example by Owen Mumford, thus use very low pressures to inject the medicament, which is typically injected through a needle in a relatively slow stream. Another self-injector includes the Simponi injector, which includes a window in the housing through which a yellow ram is visible inside a clear medicament container once the injector has been used.
Additionally, needle-assisted jet injectors have also been with higher injection forces that utilize a needle to initially penetrate the skin allowing but not restricted to an insertion depth less than that of a traditional hypodermic injector or low-pressure auto-injectors. Once the skin is penetrated with the needle, a jet mechanism is activated, causing the medicament containing liquid within the injector to be pressurized and expelled through the needle and into the skin. The injection mechanism in needle-assisted jet injectors can be configured to move the drug container and the needle forward to penetrate the skin and exert the necessary injection force to a plunger moveably disposed within the container. Alternatively, the needle and drug container can be positioned to penetrate the skin while keeping the needle and drug container in a stationary position, and the injection mechanism can be structured to pressurize the container. The pressure applied to the medicament within the injector can be less than that of a traditional jet injector, because the outer layers of the skin have already been penetrated by the needle. Similarly, the pressure applied to the medicament is preferably higher than that of a traditional auto-injector or the like, causing the medicament to penetrate the skin and be dispersed into the tissue or injected in the tissue below the skin to a depth that is sufficient so that the medicament remains substantially within the body. An additional benefit of the higher pressure includes a faster time of injection resulting in less psychological trauma to the patient and a decreased likelihood of the user inadvertently terminating the injection prematurely by removing the injector from the injection site.
Because of the stored energy associated with the trigger and injection mechanisms, accidental firing can occur due to sudden movements during shipping or due to mishandling of the device by a user including accidental actuation of the trigger mechanism. Accidental firing of the injection mechanism can cause the medicament to be expelled from the device, which can be at a dangerously high pressure, depending on the type of injection device. Further, accidental firing can cause an injection needle to move forward with respect to the device with sufficient force to penetrate the skin.
Additionally, the dimensions of many components incorporated in injectors typically constrain the design of many injectors. For example, many injectors utilize front firing-initiation mechanisms that typically require an axial translation and engagement with a triggering structure located at the back of the injector. However, this configuration typically promotes binding of the communicating triggering components due to but not limited friction between components in slidable communication and component distortion, which can be advantageous for, e.g., reducing the size of the injection device, being able to view the drug container within the device, etc.